Soil Solarization in Raised Beds

For a while I was lucky with my raised bed garden, and had few problems with pests and diseases. But this summer, a vicious case of wilt (I’m guessing Fusarium wilt) weakened or stunted several of my plants (mostly peppers) and a hungry colony of flea beetles had taken up residence in my tomatillo crop.

Combined with the drought in California, a particularly hot season, and a month-long vacation looming, I decided to put all those problems to bed — under a sheet of plastic for the remainder of the summer.

Soil solarization is a highly effective, nonchemical method for controlling soilborne diseases. While it’s commonly used on commercial farms, it’s not as prevalent in home gardens because it does require part or all of the soil to lay fallow during peak summer. In a home garden where space is often limited, it’s hard to give up a raised bed for the four to six weeks it takes to treat the soil.

But if you plan ahead (or, like me, you know you’ll be away from the garden for an extended period), soil solarization is an ideal solution for killing weed seeds, controlling nematodes and pests, eliminating soilborne plant pathogens, and improving tilth and soil biology. Think of it as a solar oven in the garden, baking everything underneath it — and what comes out is sterile soil, free of the problems that used to plague your plants.

The Benefits of Soil Solarization

Simply by using the power of the sun, solarization can rid the soil of most weed seeds, especially those from annual weeds (some perennial weeds, like Johnson grass, may have deep roots or rhizomes that the heat won’t reach).

The intense heat also speeds up the decomposition of organic material in the soil, thereby releasing soluble nutrients such as nitrogen, potassium, calcium, and magnesium and making them more available to plants.

Solarization disinfests the soil of the fungal and bacterial pathogens that cause Fusarium wilt, Verticillium wilt, potato scab, Southern blight, early blight, tomato canker, club root, crown gall, and damping off. (Note that it doesn’t work on airborne diseases, like garlic rust or late blight.) Certain fungi, such as the spores that cause Fusarium wilt, can live for many years in the soil and even in the soil clinging to your garden tools, so in these cases crop rotation is not an effective means of control.

But What About the Good Guys?

It’s believed that earthworms simply burrow deeper into the soil to escape the heat, and come back to the surface when conditions are ideal. As for beneficial soil organisms, many of them are able to either survive solarization (such as mycorrhizal fungi, which are highly heat tolerant) or rapidly recolonize the soil (such as the bacteria and fungi that parasitize plant pathogens and stimulate plant growth).

In fact, the increased numbers of beneficial microbes can make the soil more resistant to pathogens after solarization (as opposed to nonsolarized or fumigated soil). In turn, plants grow faster, get stronger, and stay healthier.

Timing Is Everything

Soil solarization works by trapping radiant energy from the sun under a thin plastic tarp to heat the soil at temperatures high enough to kill soil organisms.

Soil temperatures of at least 99°F, held steady for about four weeks, will prevent the emergence of many annual weeds and mesophilic fungi, which are the majority of common plant pathogens. That means soil solarization works best in the hottest month of the year when days are long and skies are clear, which for many zones is around the summer solstice in June or July.

Out on the Southern California coast where I live, our hottest months are September and October, when Santa Ana winds blow in from the high desert and bring us sweltering Indian summers (as well as notorious California wildfires).

Even with a relatively mild high of 75°F in early September, before the heat wave’s hit, the first 3 inches of soil in my raised bed (that gets full sun for most of the day) heats up to 116°F. In just a few weeks, that temperature will steadily rise and cook whatever’s still lingering under the plastic.

Soil solarization is less effective in the spring, even if your garden is bathed in sunshine all day. The mild weather does little to control soilborne diseases, so it’s not worth the effort to try early in the year before you start your planting.

For best results, determine the hottest four- to six-week window in your climate and plan to solarize your soil in that time.

Preparing the Soil

Before you begin, remove all the plants and mulch from the bed. If they were affected by disease, bag and trash them.

For successful solarization, the soil should be smooth and flat to allow the plastic to lay snug against it. Till or turn over the soil and remove or break up any clods, rocks, weeds, and plant debris. You don’t want anything in the soil that could potentially tear or puncture the plastic. I typically dig to a depth of a fork tine, but for heavy clay soils or soils that haven’t been cultivated in a while, going at least 1 foot deep is recommended.

Studies have shown that adding organic animal- or plant-based amendments (such as aged animal manure or cover crop residues, especially Brassicaceae cover crops like mustard) before solarizing improves its efficacy. The amendments not only increase the rate of heat generation in the soil, but also its heat-carrying capacity. And when it comes to soil solarization, more heat is always a good thing.

So, if you have some composted chicken manure, worm castings, bat guano, green manure, or well-aged kitchen compost, go ahead and incorporate that into the top 3 to 4 inches of soil, and rake it in well.

Once your soil is smoothed over, set your garden hose or irrigation system over the bed and water deeply. You want the top 12 inches of soil to be moist. The moisture conducts heat faster and deeper into the soil, while making soil pathogens more sensitive to the heat.

Choosing the Plastic

Surprisingly, clear plastic is actually more effective than black plastic at heating up soil. The sun’s rays easily pass through clear plastic, only to be trapped inside to heat the soil. By contrast, black plastic tends to absorb and deflect part of the heat.

And though it might seem counterintuitive, the thinner the plastic, the better it captures and traps the heat. Very thin plastic (1 mil) is the best option, but can be susceptible to damage from birds, critters, or the elements. A good compromise is 1.5 mil to 2 mil plastic with weatherproofing or UV resistance, as it will last long enough until you remove it from your bed.

I use a 2 mil plastic dropcloth (found at my local hardware store), but for larger gardens, you can find bulk rolls of polyethylene from farm supply stores or online.

Covering Your Raised Bed

Cut the plastic to fit, leaving at least 8 inches of overhang on all sides. There are a few ways to secure the plastic:

You can dig a trench along the inside perimeter of the raised bed and then bury the edges of the plastic 6 to 8 inches deep.

You can hold the plastic down with heavy objects (like 2x4s, pipes, planks, paving stones, or whatever you have around the house) placed around the perimeter of the bed, making sure they’re tucked tightly against the walls.

You can staple gun the plastic to the bed itself if the walls are made of wood.

Staple gunning the plastic is the way I do it, and I simply wrap the corners around the bed as if I was wrapping a package. Staples go in every 6 to 8 inches to ensure a secure seal, and then a couple of weights are laid on top of the plastic to keep it stable under any winds. (You don’t want your tarp to become a sail during a summer storm!)

The weights can be bricks, beams, sandbags, stones, or even dirt (but make sure you remove this dirt before you take the plastic off, as you don’t want to contaminate your newly sterile soil with it). You also want to be careful with sharp or rough-edged weights that could cause pressure points, leading to tears.

The idea is to keep your plastic as flat against the soil as possible, with little to no flaps or openings that could let in outside air (and decrease the temperature under the plastic). Patch up any holes in the plastic immediately with duct tape, and keep an eye on your beds throughout the four- to six-week period in case the plastic needs patching. (One morning, I found a few holes in one of my tarps that had apparently been caused by a raccoon running through the garden.)

If your daytime temperatures are cooler than normal, you can increase the amount of heat generated in your raised beds by adding a second layer of plastic over the first layer. Separate the layers slightly with PVC pipes, plastic bottles, or other smooth objects that can run the full length of the bed; that small pocket of air can increase the heat in the soil by as much as 10°F.

Maintaining the Proper Temperature

Soil solarization is most effective when the top 6 inches of soil is maintained at or above a daily temperature of at least 110°F for four to six weeks, as most pathogens reside in this upper layer. If you don’t have a soil thermometer to test this, a meat thermometer works well. I like to get a reading in the middle of the day right in the middle of the bed; afterward, I simply patch the hole with duct tape.

See all this condensation under the plastic? When I rest my hand on the surface, I can feel how how it is under there. That’s what you want, every single day.

Post-Solarization

After removing the plastic, you can sow seeds or plant transplants like normal. To avoid bringing any surviving weed seeds to the surface, stick with shallow plantings.

There’s no need to further cultivate the soil. You’re already starting fresh with soil that’s gained additional nutrients from solarization, so fertilizer can wait until midway through the season (simply do a side dressing, soil drench, or foliar spray as needed).

To improve your chances of not reinfecting the soil, always start with clean pots and new soil for seed starting and transplanting, and wash away the old (possibly contaminated) dirt from your garden tools, gloves, and other accessories that frequently come in contact with your plants.

I'm a bestselling author (my new book, The Backyard Fire Cookbook, comes out May 2019!), plant lover, and professional road-tripper (or so I like to call myself). I write about gardening, farm-to-table cooking, urban chicken-keeping, and outdoor adventuring — all that encompass a life well-lived outdoors. Beautiful Bend, Oregon, is my home.

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